What you're after is called a crowbar circuit and it's traditionally done with SCRs, as they latch. However, if you want to use a MOSFET, try it with a Zener instead of a voltage divider:I wouldn't recommend a MOSFET at all for a crowbar or any form of overvoltage protection. Power MOSFETs typically aren't specified for analogue operation, only as a switch which either on or off and the gate threshold voltage varies considerably. If the zener doesn't have a high enough power rating on its own, then add a BJT to it.
In this circuit I used a 12V Zener, but you could go with a 20V one if you want. Basically it will trigger at the Zener voltage + the N-FET's threshold voltage. So if your FET has a threshold voltage of 5V and you want the crowbar to fire at around 25V you'd use a 20V Zener. (Ideally you'd use maybe an 18V Zener to give you a bit of margin.)
In this image you can see it in action:
The fuse blows about 100ms after the crowbar triggers.
What you're after is called a crowbar circuit and it's traditionally done with SCRs, as they latch. However, if you want to use a MOSFET, try it with a Zener instead of a voltage divider:I wouldn't recommend a MOSFET at all for a crowbar. Power MOSFETs typically aren't specified for analogue operation, only as a switch which either on or off. If the zener doesn't have a high enough power rating on its own then add it to a BJT.
In this circuit I used a 12V Zener, but you could go with a 20V one if you want. Basically it will trigger at the Zener voltage + the N-FET's threshold voltage. So if your FET has a threshold voltage of 5V and you want the crowbar to fire at around 25V you'd use a 20V Zener. (Ideally you'd use maybe an 18V Zener to give you a bit of margin.)
In this image you can see it in action:
The fuse blows about 100ms after the crowbar triggers.
Thank you everybody for help and suggestions.
For me Analog is a strange language.
I know the basic, but i am focused mostly on Microprocessors.
I know i did a bo bo with the mosfet selection, but as i said is first time i touch them.
Timb, i want to ask you two questions.
First, is what software you use for simulation.
I use Proteus, but is not the greatest.
I use it because i can load a schematic an processor hex file and simulate it there.
For rest, is kindergarten.
I like your and i like to try it.
Now, about the circuit. The power supply is short circuit protected at 3A. Max voltage is 24VDC. Can you suggest me a through hole fet that can resist and i can try it before i put it on the board.
My problem i that during the assembly process in the plant, the electrician wire some times the board at 24vdc , instead of 5 vdc.
Also can you suggest me the other parts number to use, like the zenner, or value of other components.
I know i ask the moon, but i need help this time.
Thanks again to all for helm.
To T3sl4co1l
Maybe you suggest like here ? I used 3 diodes in parallel for REVERSE VOLTAGE PROTECTION 1N5819 and a 6 volts TVS SMAJ6.
To exoticelectron
I mentioned before that in rush i did a mistake and i did not read properly the data sheet, and i end up with 2N7000, but what is the suggested Through hole mosfet i can use ?
I get lost reading the parameters because it is first time i use mosfet, so what i need for my project. Do you have a part number ?
Also, how and where i should find the trigger voltage ? If i trigger it on, do i need to read any other parameters to keep it on until the fuse blow(latch it) ? My power supply, either 5 or 24VDC is short circuit protected, and the max current is 6A in the big one.
Thank you Hero999,Yes.
A complete different solution. And according with your LTspice graphs seems to work.
By "adding a much faster transient suppression diode, such as the 1N6276A, in parallel with the whole the circuit." you mean replacing D1?
In your opinion, what do you suggest as being the most reliable and cost effective ? Transistor or SCR ?SCR because it acts as a switch with a sharp on action and is very rugged compared to transistors.
My processors, mostly from Microchip support up to 6.5V, but i prefer to have the whole system to blow the fuse at 6.1VDC.Is 6.5V the absolute maximum rating or operating voltage? Most parts rated for 5V will tolerate 7V for a short length of time.
Any changes to do here to achieve this limit ?
OK Hero999,I didn't answer your question on the MOSFET because I wouldn't recommend using a MOSFET. If you do go for this option, you want something in a large case, with the appropriate voltage rating. The current rating an on resistance aren't very important.
I am still waiting and answer from Timb about the part number of the used mosfet, but yours I will try it next week and let you know the real life.
About the processor ,6.5 it is the MAX value of VDD
"Voltage on VDD with respect to VSS .... -0.3V to +6.5V" from datasheet.
VDD is the positive side and VSS is the negative side, usually called ground , or digital ground and is 0.
Can you post please your LT file ?
Thank you
for this "500mOhm resistor" do you mean milliohms or megaohms.
I know it might look like stupid question but i need to know.
Thank you
Attached is the LTSpice file. It looks different than the picture I posted previously because there's no way to embed symbols so had to use a generic 8 pin DIL for the TL431. If it doesn't work on your PC, it'll be because a model is missing. If so, let me know and I'll fix it.If you make a new folder for the project save the schematic to it and copy the symbol (.asy) to the same folder as the schematic (.asc), and also put the model (.sub, .lib. or .mod, or hierarchical .asy) there, and use the Top Directory dropdown in the Component browser to add the parts from the current folder when creating the schematic, instead of your original copies you have added to the installed component library, then zip the project folder (after cleaning unwanted generated files), the result is *ENTIRELY* portable.
.MODEL BC327S PNP(IS=0.230E-12 ISE=39.284E-15 ISC=0.118E-15 XTI=4.800
+ BF=504.327 BR=23.0 IKF=0.780 IKR=0.195 XTB=1.700
+ VAF=26.0 VAR=4.500 VJE=1.0 VJC=1.0
+ RE=0.203 RC=0.262 RB=0.800 RBM=0.400 IRB=0.100E-3
+ CJE=57.177E-12 CJC=28.600E-12 XCJC=0.650 FC=0.750
+ NF=1.0 NR=1.002 NE=1.944 NC=3.025 MJE=0.470 MJC=0.520
+ TF=0.664E-9 TR=2.600E-9 PTF=1.0 ITF=0.250 VTF=2.0 XTF=6.350
+ EG=1.110 KF=1E-9 AF=1 MFG=SIEMENS)
.SUBCKT S4025L 1 2 3
* TERMINALS: A G K
Qpnp 6 4 1 Pfor OFF
Qnpn 4 6 5 Nfor OFF
Rfor 6 4 500MEG
Rrev 1 4 500MEG
Rshort 6 5 15
Rlat 2 6 2.64
Ron 3 5 9.4m
Dfor 6 4 Zbrk
Drev 1 4 Zbrk
Dgate 6 5 Zgate
.MODEL Zbrk D (IS=10F IBV=1U BV=400)
.MODEL Zgate D (IS=10F IBV=100U BV=10 VJ=0.3)
.MODEL Pfor PNP(IS=10P BF=3.5 CJE=200p CJC=200p TF=0.3U)
.MODEL Nfor NPN(IS=10P ISE=1E-9 BF=100.0 RC=13.6M CJE=1000p CJC=200p TF=0.3U)
.ENDS
igeorge, you can use my last circuit with the single TL431 and simply replace the MOSFET with an SCR. That would work equally well, if not a bit better.Then you have a similar circuit to mine, plus an extra PNP transistor, which helps because it takes the variance in SCR trigger voltage out of the equation.
I think Hero999's circuit is way, way too complex for what it does.That's because it shows two methods of accomplishing the same thing. It may not be necessary here but it's often a good idea to have both non-latching, as well as a hard latching crowbar form of overvoltage protection.
That said, if you're intent on using a MOSFET, it'll still work fine. In this specific case, considering you have a current limited power supply and a fast blow fuse, an appropriate MOSFET should be able to crowbar the circuit without damaging the MOSFET. You can always add a 500mOhm resistor to the FET to limit current, which will prevent damage if your power supply has a large amount of capacitance on the output.A resistor may help but I don't see how the MOSFET in your circuit remains fully switched on. When the power supply voltage falls, the gate voltage will also fall, turning it off, until it reaches equilibrium.
Yes, m = milli and M = Mega but watch out for LTSpice which is not case sensitive and treats M as mill, so you have to use Meg for Mega.for this "500mOhm resistor" do you mean milliohms or megaohms.
I know it might look like stupid question but i need to know.
m=milli
M=Mega
500 milliohms, yes. :)
I try it and is missing the Tecor SCR S4025LYes, you can just use the part of the schematic with the SCR and ignore the transistors. If you want fast acting remove C1.
Do you have it ?
Also is missing definition for model "BC327S"
The capacitor is ceramic or tantalum ?
To be clear in my mind, i will use just the part of the schematic with SCR, and ignore the part with transistors.
Is this correct ?
Sorry for so many questions, but i want to see it working and move back to my programs.
If you make a new folder for the project save the schematic to it and copy the symbol (.asy) to the same folder as the schematic (.asc), and also put the model (.sub, .lib. or .mod, or hierarchical .asy) there, and use the Top Directory dropdown in the Component browser to add the parts from the current folder when creating the schematic, instead of your original copies you have added to the installed component library, then zip the project folder (after cleaning unwanted generated files), the result is *ENTIRELY* portable.I did think about posting a zip but quite often people are wary about downloading zips from unknown sources because they can contain anything. Having it all embedded in one asc file makes life much easier, until someone like me makes a mistake and they have to hunt for models.
Thank you Ian.Because huge currents can flow before the fuse blows which is not instantaneous. Everything in the current path of the fuse needs to be able to pass the huge short circuit current, at nearly the power supply voltage, until the fuse blows.
Founded did it and posted.
I can sleep well now.
I will play with it tomorrow morning.
But i rephrase again my previous question.
If the fuse is fast blowing fuse, only 0.3Amps, why i need the over rated components to work with 10-20 amps, or 3-5Wats ?
I believe the fuse will blow before the component.
Am i wrong ?
[/quote]igeorge, you can use my last circuit with the single TL431 and simply replace the MOSFET with an SCR. That would work equally well, if not a bit better.Then you have a similar circuit to mine, plus an extra PNP transistor, which helps because it takes the variance in SCR trigger voltage out of the equation.QuoteI think Hero999's circuit is way, way too complex for what it does.That's because it shows two methods of accomplishing the same thing. It may not be necessary here but it's often a good idea to have both non-latching, as well as a hard latching crowbar form of overvoltage protection.QuoteThat said, if you're intent on using a MOSFET, it'll still work fine. In this specific case, considering you have a current limited power supply and a fast blow fuse, an appropriate MOSFET should be able to crowbar the circuit without damaging the MOSFET. You can always add a 500mOhm resistor to the FET to limit current, which will prevent damage if your power supply has a large amount of capacitance on the output.A resistor may help but I don't see how the MOSFET in your circuit remains fully switched on. When the power supply voltage falls, the gate voltage will also fall, turning it off, until it reaches equilibrium.
A better solution may be one of Linear Tech's Ideal Diode/Over Voltage Protection parts. Basically, these place a MOSFET inline with your power supply input and monitor the voltage, if it goes above a set threshold it will control the gate of the FET and either shut down the output *or* try to regulate the output (like a linear regulator), depending on the part.It sounds like a similar thing could be implemented using the TL431. Obviously it won't be as fast and won't protect against reverse voltage, unless another MOSFET is added.
:(A TRIAC could be used but the circuit would need to be modified. The part number is not important. Anything which can pass the current for long enough to blow the fuse will do.
I have to comply.
Ian suggested to use a Triac instead of SCR like here
"P.S. its better to use a TRIAC as they can be triggered by current OUT of the gate, then you can directly drive it from the TL431 cathode, as any resistance or other load in the anode circuit of a TL431 adds negative feedback and makes its set point rather mushy. See Figure 24. TRIAC Crowbar in the OnSemi TL431 datasheet You can use a SCR but it needs an inverting driver as used in Hero999's 2N3055 baced circuit - just drop in a S4025L in place of the 2N3055! See Figure 25 in the TL431 datasheet."
Data sheet https://www.onsemi.com/pub/Collateral/TL431-D.PDF (https://www.onsemi.com/pub/Collateral/TL431-D.PDF)
Any suggestion for part number to test for Triac?
To Ian and Hero:I'm not aware of any fuse models for LTSpice. I don't see any benefit in simulating it. because it's something which varies quite widely, depending on the temperature and there's quite a wide variance from fuse to fuse. If you know the short circuit current, then you can predict whether the fuse will blow and how long it will take by looking at the fuse's datasheet.
Where i can find a fuse and put it in the LT circuit just before D1.
On the latest LT i download yesterday is no fuse symbol or component.
I would like to see the graph with a fuse on the circuit.
Can you help me on this ?
Thank you
To Timb:
Thanks Tim, but i am still waiting for your Mosfet to buy Monday.
For your MOSFET, try the STP16NF06. It's got a very nice dV/dT rating and can handle up to 50A for at least 10mS. That should be more than enough time to discharge any output capacitance on the power supply. (A power supply should, ideally, have less than 100uF on the output.)
Add a wirewound 500mOhm resistor to the drain of the FET, rated for 2W or 3W (wirewound resistors have high peak surge current capability).
Based on some tables from fuse data sheets, a 1A fast blow fuse shouldn't have any trouble opening before the MOSFET is damaged. You could also add a small heatsink to the FET if you need to extend the pulse duration.
I can also get you a part number for an SCR that would be a drop in replacement for the MOSFET in my circuit, if you want.
Thanks Hero, i missed.As this is running off 5V, the lower threshold is better.
It comes in 2 flavors
vgs(th) 2.5 v or 4 v
Any preference?
To Ian and Hero:
Where i can find a fuse and put it in the LT circuit just before D1.
On the latest LT i download yesterday is no fuse symbol or component.
I would like to see the graph with a fuse on the circuit.
Can you help me on this ?
Thank you
You see, my mind is simple. For me, in digital is 0 or 1For this particular kind of circuit:
Thank you ALL for helping me on this tedious task.You could but the gate trigger voltage is highly variable.
I still have a dumb question.
As i explained through this post, analog for me it is a strange territory.
Using basic resistors, capacitors or OpAmp, i did , the rest, is somewhere in the dark.
Here is the question:
We use the TL431 to trigger the SCR, and a resistor divider to trigger TL.
How about using just Resistor divider to trigger the SCR, without using TL ?
To make things clear, the fuse it is not polyfuse. It is a one time fast blowing fuse 0.3A.I've already posted that circuit, with appropriate resistor values for 6.5V.
The maximum current on the board is 200 milliamps.
All the IC are rated to Max 6VDC.
We do not want to use polyfuse for the simple reason, that who made mistake and blow the fuse, has to go to his supervisor and get a replacement (electricians do not like to do that). In this way we can make them to look twice before putting power.
Based in fig24, and your choice of Triac, can you please suggest me the value of the 3 resistors and a part number for triac.
To test it first on the breadboard i need a through hole version.
Later i will find a similar or the same on smd.
I read the formula to calculate the Vout, but i do not know what should be.
The whole circuit should go on short for 6 VDC.
Once the triac start conducting, the fuse will blow and everything will be safe.
I put a micro to flash a led on the breadboard on 5VDC and when i have the protection i will supply 24 and pray.
The TRIAC + TL431 is a close tolerance crowbar. Its easy to hold the trip point to 2% tolerance if you use a 1% TL431A and 1% resistors, and fractionally over that for a 2% TL431 and 0.1% resistors, so with the trip point set at 5.8V, you can comfortably stay under the AD8541's abs max supply voltage rating of 6.0V and the PIC16F882's abs. max. supply voltage rating of 6.5V without a significant risk of nuisance tripping due to normal fluctuations of the 5V railYes, the TRIAC + TL431 will provide
However your chosen SMAJ6.0CA TVS diode has a breakdown voltage range of 6.67V to 7.37V, so will *NEVER* provide adequate protection for the PIC and the OPAMP as they are guaranteed to see more than 6.6V before the fuse blows. Depending on the peak fusing current, Vdd could easily go over 8V, as the only guaranteed clamping spec is 10.3V @38.8A pulse.You're right there. I think it survived so well because the overvoltage was very brief and I believe manufactures often specify their parts very cautiously. It wouldn't surprise me if a simple MCU can stand around 8V for a few hundred ms with no lasting damage.
Thank you again to all of you.Oh I understand why you need a one shot system which can't be reset by someone who doesn't know what they're doing. You need definitive proof that the electrician is making mistakes. If you went for a PTC fuse, then repeated overvoltage could still damage the components, causing reliablity problems later and you'd have no way of proving it.
Let me explain again the scenario.
A machine builder build a machine where he got 2 sources of voltage; 5 VDC and 24 VDC.
He buys boards from a supplier which test them before and guarantee that they are good.
The electrician wiring the machine can do one of the mistake:
1.Wire 5 VDC in reverse polarity; solved by the reverse Diode on the input AFTER the fuse. Result = blow the fuse, no damage.
2.Wire 24VDC in reverse polarity. The solution and result is the same as above.
3.Wire correctly 24 VDC. Solution is the proposed TVS (still needs more testing). Result is = Blow the fuse.
What ever the electrician do wrong will be discovered on the first test run.
Assuming a board is destroyed, it will be replaced and the voltage applied correctly ; 5 VDC.
The machine is usually under test for 1-2 weeks before it will leave the shop and go to customer. During that time, what ever can be wrong it might happen, but it will be corrected immediately.
The reason do not put a polyfuse, or socket fuse is to prove to machine builder that the boards were destroyed during assembly or tests.
In other words the seller of the boards cover his a..
During production in the plant, it is no way that somebody will go and reverse polarity or change the voltage supply.
Like i said, for future projects, the triac solution will be incorporated on the price. For the existing contract , the TVS is the most acceptable solution.
The power supply is big enough to supply a full 24 VDC to the whole machine.That's good: you can be sure the fuse will certainly trip very quickly, just make sure the fuse has adequate breaking capacity for the maximum short circuit that beefy PSU can deliver. A ceramic fuse will give better protection than a glass fuse, which would be borderline in this case.
It is in the range of 20-30 A. It is overrated.
The wires are usually 16 gauge, big wires with a very low drop.
The length of the wire is no more than 30 feet.
The supply, provide power radial.
From a main terminal strip to each oh the users is one wire.
Like i said. The board builder, us, wants to prove to machine builder that the board are damaged during the assembly of the machine.
Before, they use to come back to us complaining that a number of boards does not works,WERE delivered defective.
In this way we can prevent the complains, and get paid for repairing the board or replacements.